Aim: The aim of this study was to investigate hydration state and perceptual response of “sumo” wrestlers after intake of water or oral rehydration solution (ORS) during training in a hot environment.Methods: Eight male participants (a university sumo wrestling club) in the study ingested water (Wat-D) or ORS (ORS-D) during sumo training in a randomized crossover manner. The environmental temperature condition during training was over 20 °C at a wet bulb globe temperature. Body weight, water distribution (bioelectrical impedance analysis), vital signs, hematological variables, serum osmolality, total protein (TP), albumin (Alb), lactic acid, sodium, potassium, and chloride were measured. Visual analogue scale (VAS) for fatigue and thirst were compared between the two drinks.Results: Body weight did not change remarkably after training with Wat-D and ORS-D. Total body water (TBW) and extracellular fluid (ECF) volumes after training were significantly greater with ORS-D than Wat-D. Hematocrit, TP, Alb, serum electrolytes, and serum osmolality changes after training were smaller with ORS-D than Wat-D. VAS scores for fatigue and thirst were not significantly different between the two drinks.Conclusion: After ORS consumption, TBW and ECF volumes were maintained for a prolonged time after training compared with those after water intake, and changes in serum osmolality, TP, Alb, and serum electrolyte levels were slight, indicating excellent water retention after consumption of ORS. Based on the results, since the maintenance of hydration status is particularly important for physical condition, ORS intake during exercise in a hot environment is considered effective and useful.